This invention relates generally to the field of absorption chiller systems, and more particularly to a method of controlling the leaving chilled water temperature of the system.
One of the inherent control problems of an absorption chiller is that its thermodynamic properties create a slow moving cycle. This slow response to dynamic building loads is further amplified by a unique valve position vs. fuel consumption curve. That is, because of variations between installations, a curve relating the position of the capacity valve to the heat input for all installations is impossible to obtain. To overcome this problem, when the unit is first installed in the building, the service technician adjusts the capacity valve to the minimum and maximum heat input values. Between these two points, the combustion characteristics are adjusted for a xe2x80x9cclean burnxe2x80x9d for a gas installation, i.e., adjusted to meet various pollution control requirements. This adjustment is dependent on several variables unique to the specific installation site; thus, the adjustment is done on-site at the time of installation. Only a few data points relating the position of capacity valve to the heat input are known at the time of installation, so that the relationship between the valve position and the heat input is known only as either an assumed linear curve or as a step function. The data points have to be determined empirically during installation.
The controls that regulate the movement of the capacity valve have no feedback other than the leaving chilled water temperature to determine the valve position, which controls the heat input to the system. The combination of the unique non-linear combustion curve and a slow moving cycle is one component of a problem termed xe2x80x9ccapacity valve huntingxe2x80x9d, which is an undesirable effect that causes oscillations in the leaving chilled water temperature. The system adjustments are either too much or too little, so that the actual leaving chilled water temperature oscillates around the setpoint.
Briefly stated, data points are determined for an absorption chiller system which relate a position of the capacity valve to the heat input into the system. A continuous curve is determined which estimates the relationship between the position of the capacity valve and the heat input for all of the data points and all the points in between. The slope of this curve is the valve gain. The error for the system is defined as the difference between the setpoint and the leaving chilled water temperature. The leaving chilled water temperature of the system is measured to determine the actual error for the system, after which a linearizing gain derived as a function of the inverse of the valve gain is used in the system control algorithm to linearize the overall valve gain, thereby eliminating capacity valve hunting and producing an improved transient response.
According to an embodiment of the invention, a method for calibrating a capacity valve for an absorption chiller system includes the steps of (a) empirically determining a plurality of data points for said system that relate a position of said capacity valve to heat input into said system; (b) determining a continuous curve which estimates a relationship between said position of said capacity valve and said heat input for all of said plurality of data points and all points therebetween; (c) measuring a leaving chilled water temperature of said system; (d) defining an error for said system as a difference between a setpoint and said leaving chilled water temperature; (e) determining said error for said system; and (f) using a function of said relationship in a control algorithm for said system to reduce said error.
According to an embodiment of the invention, an absorption control system for an absorption chiller includes means for empirically determining a plurality of data points for said chiller that relate a position of a capacity valve to heat input into said chiller; means for determining a continuous curve which estimates a relationship between said position of said capacity valve and said heat input for all of said plurality of data points and all points therebetween; means for measuring a leaving chilled water temperature of said chiller; means for defining an error for said chiller as a difference between a setpoint and said leaving chilled water temperature; means for determining said error for said chiller; and means for using a function of said relationship in a control algorithm in said control system of said chiller to reduce said error.